1. Technical Field
The present invention relates in general to a method and apparatus for wireless networking. The invention more particularly relates to wireless networking router systems for facilitating communications via wireless transmission, in a more efficient and reliable manner.
2. Background Art
In industrial automation it becomes necessary to gather information from several remote sites and use that information to control operations at the remote sites. The data is gathered by using various types of transducers that measure the physical variables (such as temperature, revolutions per minute, etc.) and convert them into electrical signals. Similarly the remote equipment can be controlled by using mechanisms that convert electrical signals into physical motion such as turning a valve or operating a switch. A Remote Terminal Unit (RTU) is the interface between these electrical signals and a communications medium. Remote Terminal Units are also known as Logic Controllers and Programmable Logic Controllers (PLC).
An RTU will convert the electrical signals into digital representations and send them on a digital communications channel. They will also receive digital data from the communications channel and convert it into electrical signals for control of the equipment at the site. Various communications protocols have been invented to carry the data on various types of communications channels. The most typical communications channel is a multidrop asynchronous serial channel with a “master” station that controls access to the channel and several “slave” stations that communicate, one at a time, with the master station. An RTU would be one of the slave stations and a central data processor would be the master. Several communications protocols, such as MODBUS, have been invented to handle this architecture. This type of protocol typically has the data gathering program at the master computer site request data from one slave station at a time and send data to one slave at a time. The control program has complete control over the activity on the communication channel.
Occasionally the situation exists where it is difficult to place all of the RTUs on a wire channel. This most often happens when the distances between the RTUs are too great for wire connections. It also happens when there are physical or economic or legal barriers to making the wire connections. In these cases a wireless connection is needed. Since the operational characteristics of radios are very different from those of wires, the use of radio has exposed serious weaknesses in the master/slave model of communications. The master/slave communications model assumes that there is a master that can control a single channel that is also available to all of the slaves.
It is frequently impossible or at least very difficult, to get a radio signal from a central master site to all of the slave sites. Even when there is a place where a master station could be put so that it can communicate with all of the slave stations, this place is frequently not a good place to locate the data gathering program (such as a mountain top.) The normal solution to these problems involves a series of repeater stations, all still under the control of the data gathering program. In essence extra radio channels have been added, but are being used as if they were a part of a single channel. This means that when one of the channels is operational other channels must be still, even when the separate channels do not conflict in any physical way with each other.
Internet Protocol has attempted to solve this problem by removing the control of the networks and links from the application (such as the data gathering program.) The application program sends data to the address of the destination and is not concerned with the processing and communication necessary to actually move the data. All of the channels can be used to their capacity without impacting the application program.
Recently some RTUs have begun to use the Internet Protocol (IP) using ethernet communications. Using IP protocol allows having a router on the same ethernet. An IP router, such as the one marketed under the tradename “MAVRIC” by Metric Systems Corp., of Carlsbad, Calif., can allow access to wireless transmission of the data across a network of arbitrary topology, by interconnecting ethernet or other inputs to the flexibility of the topology offered by IP routing allows placement of radios wherever they are needed for connectivity. The combination of an RTU using IP on an ethernet channel and an IP router on the same ethernet channel solves all the topological problems of using wireless communications for data acquisition and control.
While such a system has proven highly successful, it would be desireable to have a more band width utilization efficient system to operate at a variety of different bands.